I. Field of the Invention
This invention relates to a method for obtaining image data with a tomographic apparatus. More specifically, this invention relates to a method for obtaining calibrated image data, having reduced noise, for a target object in order to reconstruct a tomographic image of the target object.
II. Background Information
Conventional methods for obtaining image data for a target object with a tomographic apparatus include steps for calibrating the image data of the target object against image data obtained for a reference object. This calibration is performed in order to eliminate, from the image data for the target object, non-object-intrinsic information which may be contained within the image data as a result of the techniques utilized to obtain the image data using the tomographic apparatus.
Typically, a tomographic apparatus includes a radiation source for generating radiation beams, and a chamber through which these radiation beams are directed. The chamber of the tomographic apparatus includes a portion specifically adapted for receipt of an object. The tomographic apparatus further includes a plurality of individual detectors located opposite the radiation source for detecting radiation emerging from the chamber at various points, a data aquisition unit and a memory for storing data.
Image data for the target object may be obtained by placing the target object within the portion of the chamber adapted for receipt of an object, and subsequently subjecting the target object to radiation beams from the radiation source. The radiation from the radiation source will pass through the object and emerge from the chamber so as to be detected by the detectors positioned at various points opposite the radiation source. The intensity of each radiation beam emerging from the chamber will vary in accordance with the composition of the target object at the points through which each radiation beam has passed. Each detector responds to the emerging radiation detected at a given point by generating a response signal for the radiation detected at that point. The response signals are transferred to the data acquisition unit of the tomographic apparatus which generates image data for the target object. The image data is stored in the memory.
Unfortunately, for a variety of reasons the composition of the target is not accurately reflected by the emerging radiation which is detected by the detectors when the target object is placed in the chamber portion and subjected to radiation, i.e., scanned. Accordingly, the image data generated when the target object is scanned is inaccurate and not ideal for reconstructing a tomographic image of the target object.
One reason why the image data obtained by scanning the target object is not ideal is that a portion of the radiation used to obtain the image data is scattered by the target object. Scattered radiation is radiation which is not attenuated in proportion to the composition or density of the target object, and which does not emerge from the chamber at a position opposite the radiation source. Rather, scattered radiation is deflected by the target object, and emerges from the chamber at some position other than opposite the radiation source. A beam of radiation may be deflected by points inside the target object more than once or may be scattered by another radiation beam. A given detector detects undeflected beams emerging from the oppositely positioned radiation source along with deflected radiation incident on the detector. The detector cannot distinguish between deflected and undeflected radiation. The reconstruction mathematics, presumes, however, that the detector records only the radiation which is undeflected, and not also radiation which has been deflected. As a result, scattered radiation causes the generation of image data which falsely indicates dense target object composition. The typical target object scatters at least some measure of radiation.
Another reason for inaccurate image data may be the effect of off-focal radiation, i.e., the effect of radiation generated by a non-point radiation source. The generation of image data and the reconstruction of tomographic images is based on the idealization of a point source of radiation. Accordingly, if the radiation source is a non-point source, the data and reconstructed images will be to some degree inaccurate.
Yet another reason for inaccurate image data may be that the radiation from the radiation source is polychromatic X-ray radiation of mixed energy levels. The generation of image data and reconstruction of tomographic images is also based on the idealization of monochromatic radiation having a given energy level. Typical tomographic apparatus include radiation sources which are extended sources, that is, which are not point sources, and sources which emit polychromatic X-ray radiation, and typical tomographic apparatus would, therefore, produce inaccurate images from the target object image data. Other reasons for inaccurate image data such as, for example, differences between the detectors may also exist.
Present methods for obtaining image data mitigate against the effects of scattering, off-focal radiation, polychromaticity, and other undesirable effects, as those effects appear in the data obtained when scanning the target object, by also scanning a reference object and calibrating the data obtained for the target object against data obtained for the reference object. The reference object, which is chosen and scanned to obtain data for use in calibrating the data for the target object, typically has physical characteristics similar to the target object in so far as the characteristics of the target object and reference object are related to undesirable effects caused by, for example, scattering, off-focal radiation and polychromaticity.
For example, the target object may be a human patient or a portion of a human patient, the patient being scanned in order to produce a tomographic image for diagnostic use. In such a case, the reference object selected for use in calibrating the data of the target object is typically a cylinder of water having a size substantially similar to the size of the patient, i.e., to the "field size" for the target object. The physical characteristics of water are substantially similar to those of the human patient. Both the human patient and the water cylinder demonstrate substantially similar responses to off-focal polychromatic X-ray radiation, and both scatter radiation in a similar manner. Accordingly, by differencing the data obtained when scanning a water cylinder and a patient, the inaccuracies resulting when the human patient, that is, the target object is scanned and which are reflected in the image data for the human patient may be minimized, and more accurate calibrated image data obtained.
The water cylinder used when obtaining image data for a human patient is typically of uniform dimensions (wall thickness and diameter) throughout. The data generated for the cylinder is adjusted to achieve a flat or equivalent set of signal responses from the detectors, i.e., a flat profile. In order to achieve a flat profile, a compensation object, typically an aluminum wedge, which is concave where the water cylinder is convex, may be inserted in the tomographic apparatus chamber at a location between the radiation source and the portion of the chamber where the human patient would be placed.
Data obtained when the water cylinder is scanned, data obtained when the patient is scanned, and the water cylinder-calibrated image data or difference data, which is actually used to reconstruct a tomographic image of the patient, are stored in the memory of the tomographic apparatus. The tomographic apparatus also comprises a data processing unit, and differencing of the data for the water-cylinder scan and for the patient scan is performed by the data processing unit.
Each time a patient is scanned, a fresh set of data or profile is generated and stored. There is, however, much inconvenience associated with physically mounting water cylinders that is, with placing water cylinders in the portion of the chamber adapted for receipt of objects, and scanning the water cylinders. Therefore, a water cylinder scan is not performed each time a patient scan is performed. Instead, water cylinder scans are taken for various field sizes (e.g., five (5) different field sizes) and resulting water cylinder profiles are stored for use in obtaining calibrated image data for various patient scans.
The stored water cylinder profiles remain valid and useful for calibrating patient scan data only for a finite period of time, however. As various operatings parameters of the tomographic apparatus change, as when any alteration is performed in the radiation source optical assembly or with the detectors, the water cylinder profiles must be regenerated. Data for the water cylinder scan, that is, water cylinder profiles must also be regenerated to adjust for normal operating parameter changes of the tomographic apparatus not brought on by systematic alterations of the kind mentioned above. Accordingly, under the present methods utilized for obtaining image data, the inconvenient steps of physically mounting and scanning a water cylinder must be repeated with frequency as long as it is desired to reconstruct a tomographic image of a target object using calibrated image data, calibrated using a reference object with physical characteristics similar to those of the target object. This is so, notwithstanding the ability of the tomographic apparatus to store data.
In addition to the problem of repeated inconvenient mounting and scanning of water cylinders, further problems are encountered when using conventional methods to obtain calibrated image data for a patient using a water cylinder. One such problem is the high load which is placed on the radiation source when producing radiation of sufficient intensity to generate water cylinder profiles. A water cylinder causes significant attenuation of the radiation beams passing through the tomographic apparatus chamber and a water cylinder scan thus necessitates the production of high intensity radiation by the radiation source.
The noise present in the data obtained from scanning an object varies in accordance with the square root of N, .sqroot.N, where N is the number of counts, or photons of radiation detected from a beam after it emerges from an object. The signal is directly proportional to the number of counts N. Accordingly, the signal to noise ratio is given by N/.sqroot.N=.sqroot.N, and noise increases as N increases. Since the water cylinder scan involves the production of high intensity radiation, that is, a high N value, the proportion of noise in data obtained from a water cylinder scan is high. The amount of noise present in the image data for the target object obtained using the water cylinder data is, therefore, also high, with the signal to noise ratio for the image data being undesirably low.
The signal to noise ratio for the image data may be improved by using a technique such as averaging the data from several water cylinder scans. The noise present in an averaged water cylinder profile decreases in proportion to the number of scans employed to obtained the averaged profile.
An important factor when obtaining the data from several water cylinder scans taken while rotating the radiation source, however, is the position of the water cylinder in the portion of the chamber for objects. If the water cylinder center is coincident with the center of the portion of the chambers, then as the radiation source is rotated the radiation source will remain the same distance from the water cylinder, and will thus cause the generation of water cylinder data for each detector which may be averaged with other data for the same detector. If, on the other hand, the water cylinder and portion centers are not coincident, distances between the source and the water cylinder for each of the several scans will vary. Data generated for each detector will be based on radiation travelling different distances from the radiation source to the water cylinder, and the data for each detector will, therefore, not be fit for averaging with other data for the same detector.
Conventional methods for averaging water cylinder profiles do not account for the uniqueness of the data generated by a detector as a function of the source-to-cylinder distance, however. This is true of conventional averaging methods even though the liklihood of placing a cylinder in the portion of the chamber so that the water cylinder and portion have coincident centers is quite low.
Conventional methods for averaging also fail to account for the varying sensitivities of the detectors of the tomographic apparatus as a function of the changed angular placement of the detectors relative to the water cylinder, i.e., angular sensitivity. Additionally, each water cylinder scan taken in order to obtained averaged water cylinder scan data requires the placement of a further high load on the radiation source.
Accordingly, an object of the present invention is to provide a method for obtaining calibrated image data for a target object with a tomographic apparatus, wherein the method does not require repeated scanning of a reference object to obtain data for use in calibrating the image data for the target object.
An additional object of the present invention is to provide a method for obtaining calibrated image data for a target object with a tomographic apparatus, wherein the image data for the target object is calibrated against calibration data which is updated in accordance with changes in operating parameters for the tomographic apparatus.
A still further object of the present invention is to provide a method for obtaining calibrated image data for a target object with a tomographic apparatus, wherein the method enables the target object image data to be calibrated against data obtained without placing a high load on the radiation source, and against data which still accounts for the type of scattering, off-focal radiation, and polychromatic effects which result when scanning the target object.
Yet a further object of the present invention is to reduce the noise present in the calibrated image data used to reconstruct a tomographic image of the target object.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention.